An envelope detector is an electronic circuit that may be used to detect an amplitude envelope of an AC input signal. For example, an envelope detector may generate a DC output signal that is proportional to an amplitude envelope of an RF input signal. Applications for envelope detectors include generating a feedback signal for controlling the power of an RF transmitter.
An envelope detector may generate a DC output signal by performing a half wave rectification on an AC input signal using a diode rectifier. A diode rectifier in an envelope detector may be biased so that it responds to a relatively small amplitude AC input signal. For example, a bias current may be applied to bias a diode rectifier at its threshold of conduction. Unfortunately, a bias current applied to the diode rectifier of an envelope detector may generate an undesirable DC offset in its DC output signal.
An external differential amplifier may be employed to subtract off the undesirable DC offset from the DC output signal of an envelope detector. For example, an envelope detector may include a dummy diode for generating a reference DC offset that mirrors the DC offset produced by the diode rectifier. A differential amplifier external to the envelope detector may then be used to subtract the reference DC offset from the DC output signal.
Unfortunately, a differential amplifier for subtracting a reference DC offset from a DC output signal may increase the overall cost of an envelope detector. In addition, an envelope detector implemented in an integrated circuit chip package may consume an additional input/output pin of the chip package to provide the reference DC offset to an external differential amplifier.